Hydrostatically steered vehicles have been around for many years and their application to windrowers has been significant. There has been a limit to the transport speed of these windrowers due to instability at high speed with the drive wheels ahead of the castor wheels (cab forward). Higher speeds on the road allow the user to cover large distances without the use of trailers and tow vehicles. It has been found that if the machine travels with the castor wheels leading the drive wheels (engine forward), the machine is much more stable and higher speeds are possible.
While in a turn with a self propelled machine that has the steered wheels leading, the centrifugal force at the center of gravity is such that it tends to assist the rear driving wheels in straightening out the machine. On the other hand when in a turn with a self propelled machine that has the driving wheels leading, the centrifugal force at the center of gravity is such that it encourages the turn, and the front driving wheel must overcome this force in order to straighten out the machine. Thus the machine is more stable, or wants to travel more in a straight line, when the steered wheels are leading and the driving wheel are trailing.
Other bi-directional vehicles such as the Ford TV140, although hydrostatically driven, are not hydrostatically steered. The TV140 machine relies on the pivoting action between front and rear sections of the tractor to steer. Control of the TV140 machine is done using hydraulic valving and cylinders.
Coop Implements manufactured the model 742 windrower tractor which is bi-directional. This was done to allow the header to be detached and connected at the drive wheel end of the tractor. This also provided weight to the drive wheels to make the machine more controllable during transport. However, the purpose for traveling in the engine forward direction was not to be able to increase transport speed, as the machine traveled only 12 mph, the same as it would have in cab forward direction. Also, the controls were not mounted to the seat base, so they had to be operated with the left hand instead of the right as would normally be the case in cab forward direction.